As a rotation mechanism in which a rotor that is driven to rotate around its axis is accommodated in a casing thereof, there is a centrifugal compressor which compresses gas using a centrifugal force. As the centrifugal compressor, a so-called barrel-type compressor having a cylindrical casing and a so-called split-type compressor having a casing that can be split into two portions are known (for example, refer to PTL 1). Here, in the barrel-type compressor, components other than the casing, that is, an internal unit having a rotor, a bearing, a seal member, and the like that are integrally configured are accommodated. In a case where maintenance in the inside of the barrel-type compressor is performed, by pulling out the internal unit from one end opening of the cylindrical casing, the components in the inside thereof can be collectively replaced. The barrel-type compressor has a high internal airtightness and thus is likely to be applied to centrifugal compressors having a high internal pressure.
On the other hand, in the split-type compressor, when the casing on the upper side among the casings which can be split into two portions is detached, the bearing and the seal member are removed along with the casing on the upper side. Accordingly, the rotor and the like inside are exposed, and the maintenance in the inside can be performed at a place where the compressor is installed. In the split-type compressor, since the casings can be split into two portions, compared to the barrel-type compressor, the internal airtightness is poor, and thus the split-type compressor is likely to be applied to centrifugal compressors having a low internal pressure.
However, as a sea compressor used in the facilities which refine petroleum or natural gas on a ship, the barrel-type compressor is mainly used. This is because it is difficult to perform maintenance in the inside of the compressor on the sea where only a limited space and a minimum number of personnel can be ensured. Therefore, the barrel-type compressor which can be easily maintained by collectively replacing the components in the inside thereof is appropriate.